Prosecutors launch investigation after Genoa's bridge collapse as search teams comb through the rubble for victims.

Italian emergency workers pulled more bodies out of tons of broken concrete and twisted steel on Wednesday after a highway bridge collapsed in Genoa, raising the death toll to at least 39 people.

A 50-metre section of the Morandi Bridge - part of a toll highway and one of the main arteries into the port city - collapsed at about 12pm local time on Tuesday, taking down with it about 35 cars and three trucks that had been crossing at the time.

Civil protection authorities confirmed Wednesday that 39 people had died and 15 were injured. Interior Minister Matteo Salvini said three children were among the dead.

Three people were missing and about a dozen remained hospitalised in serious condition, authorities said.

Working with heavy equipment, rescuers climbed over concrete slabs with sniffer dogs searching for survivors or bodies.

The head of Genoa's public prosecutor's office, Francesco Cozzi, said he was ready to "open a file for negligent multiple homicide and disaster", and it would be against unknown persons "because the reasons for the tragedy are still unknown".

Engineering experts, noting the bridge was 51 years old, said corrosion and weather could have been factors in its collapse.

Mehdi Kashani, an associate professor in structural mechanics at the University of Southampton in the UK, said pressure from "dynamic loads," such as heavy traffic or strong winds, could have resulted in "fatigue damage" in the bridge's parts.

Bridge operator Autostrade per l'Italia said it had carried out regular, sophisticated checks on the structure before the disaster and these had provided reassuring results.

Italy's transport minister ordered safety checks of all major infrastructure across the country. Danilo Toninelli also called on Wednesday for senior managers to resign at the company operating the bridge.

The government will also look into stripping Autostrade per l'Italia, a unit of the Atlantia group, of the concession to manage the motorway that included the Morandi Bridge, and imposing financial penalties on the group, Toninelli said.

The Morandi Bridge was built in the 1960s on the A10 toll motorway connecting Genoa to French border.

"Autostrade per l'Italia was not able to fulfil its obligations under the contract regulating management of this infrastructure," Toninelli said on RAI 1 state television.

Evacuations

Houses in the immediate vicinity have been evacuated out of fear that the two remaining sections of the bridge would also give way.

The man, who was seen walking with his arm bandaged after suffering a shoulder sprain and some minor wounds, was still in shock when cameras approached him near the scene. "I was in front of the truck, when I flew away like everything else. My truck is buried."

A footballer, Davide Capello, who had been driving on the bridge, said it was a miracle he was still alive.

"I saw the road collapse in front of me and I started falling with it. I was lucky," he said. "I didn't black out, I immediately called firefighters to come and get me."

The reasons for the collapse have not yet been established.

"Now is the time for a common commitment in order to face the emergency, assist the wounded, support those who are mourning," President Sergio Mattarella said in a statement. "Then a serious investigation into the causes of what happened [is needed]. No authority can evade full responsibility."

The 1,182-metre structure was inaugurated in 1967 and went through major reinforcement work in the 1990s.

Ansa news agency cited a 2011 report by Autostrade per l'Italia as saying the bridge had been suffering from decay.

"Queues of cars and the volume of traffic provoke intense decay of the Morandi viaduct structure on a daily basis in the rush hours as it is subject to major demands," the report said, adding the bridge was for this reason in need for continuous maintenance work.

Il Sole 24 Ore, Italy's financial newspaper, reported the company had launched a 20-million euro call for tenders in April 2018 for "structural retrofitting" of the bridge.

A bystander, Pietro M, told Ansa he saw "lightning strike the bridge" just before the collapse, as a wave of bad weather was sweeping through Italy at the time.

Transport Minister Toninelli said in a TV interview "the first major intervention by this government will be to carry out ordinary maintenance of the infrastructure that already exists".

Salvini said he wanted "names and surnames of those responsible", adding austerity measures imposed by the European Union may have led to budget cuts that caused local authorities to save on maintenance costs.